Add KeySyncTask

We should be doing all syncing on another thread, so as not to slow down
screen unlock. This is an initial skeleton implementation that does
nothing.

Test: adb shell am instrument -w -e package com.android.server.locksettings.recoverablekeystore com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I43ebe3f026cbba62985a464b6c0e0a9151448a51